1. Field of the Invention
The present invention relates to calibration of current variations in the pixels/sub-pixels of an active matrix organic light-emitting diode (AMOLED) display caused by non-uniformity of thin-film transistors (TFTs) in the sub-pixels.
2. Description of the Related Arts
An OLED display is generally comprised of an array of organic light emitting diodes (hereafter referred to as “OLED diodes”) that have carbon-based films deposited between two charged electrodes. Generally one electrode is comprised of a transparent conductor, for example, indium tin oxide (ITO). Generally, the organic material films are comprised of a hole-injection layer, a hole-transport layer, an emissive layer and an electron-transport layer. When voltage is applied to the OLED diode, the injected positive and negative charges recombine in the emissive layer and transduce electrical energy to light energy. Unlike liquid crystal displays (LCDs) that require backlighting, OLED displays are self-emissive devices—they emit light rather than modulate transmitted or reflected light. Accordingly, OLEDs are brighter, thinner, faster and lighter than LCDs, and use less power, offer higher contrast and are cheaper to manufacture.
An OLED display typically includes a plurality of OLED diodes arranged in a matrix form including a plurality of rows and a plurality of columns, with the intersection of each row and each column forming a pixel of the OLED display. An OLED display is generally activated by way of a current driving method that relies on either a passive-matrix (PM) scheme or an active-matrix (AM) scheme.
In a passive matrix OLED (PM OLED) display, a matrix of electrically-conducting rows and columns forms a two-dimensional array of picture elements called pixels. Sandwiched between the orthogonal column and row lines are thin films of organic material of the OLEDs that are activated to emit light when current is applied to the designated row and column lines. The brightness of each pixel is proportional to the amount of current applied to the OLED diodes of the pixel. While PM OLEDs are fairly simple structures to design and fabricate, they demand relatively expensive, current-sourced drive electronics to operate effectively and are limited as to the number of lines because only one line can be on at a time and therefore the PM OLED must have instantaneous brightness equal to the desired average brightness times the number of lines. Thus, PM OLED displays are typically limited to under 100 lines. In addition, their power consumption is significantly higher than that required by an active-matrix OLED. PM OLED displays are most practical in alpha-numeric displays rather than higher resolution graphic displays.
An active-matrix OLED (AMOLED) display is comprised of OLED pixels (that are each comprised of R, G, B sub-pixels) that have been deposited or integrated onto a thin film transistor (TFT) array to form a matrix of pixels that emit light upon electrical activation. In contrast to a PM OLED display, for which electricity is distributed row by row, the active-matrix TFT backplane acts as an array of switches coupled with sample and hold circuitry that control and hold the amount of current flowing through each individual OLED sub-pixel during the total frame time. The active matrix TFT array continuously controls the current that flows to the OLED diodes in each of the sub-pixels, signaling to each pixel how brightly to illuminate.
AMOLED displays require regulated current in each pixel to produce a desired brightness from the pixel. Ideally, the TFTs in the active matrix TFT array exhibit uniform electrical characteristics, so that the AMOLED display can be precisely controlled in a uniform manner. However, the TFTs in the AMOLED are typically fabricated with poly-silicon (p-Si) that is difficult to fabricate in a uniform manner. This is because p-Si is made by converting amorphous silicon (a-Si) to p-Si by laser annealing the a-Si to increase the crystal grain size. The larger the crystal grain size, the faster and more stable is the resulting semiconductor material. Unfortunately the grain size produced in the laser anneal step is not uniform due to a temperature spread in the laser beam. Thus, uniform TFTs are very difficult to produce and thus the current supplied by TFTs in conventional AMOLED displays is often non-uniform, resulting in non-uniform display brightness. TFT non-uniformity throughout the OLED display causes “Mura” (streaking or spots) in the OLED displays made with p-Si TFTs. In other words, TFTs may produce different OLED currents due to their non-uniformities from pixel to pixel, even if the same gate voltage is applied to the TFTs.
Another problem with AMOLED displays occurs due to aging of the material in the OLEDs. As the OLED diode in each sub-pixel ages with use, it becomes less efficient in converting current to light, i.e., the efficiency of light emission of the OLED diode decreases. Thus, as OLED diode current to light efficiency of the OLED material decreases with use (age), light emitted from an OLED diode in each sub-pixel for a given gate voltage applied to the drive TFTs of the OLED display also decreases. As a result, the OLED display emits less light for display than desired in response to a given gate voltage applied to the drive TFTs. In addition, since the OLED diodes on various parts of the AMOLED display do not age (are not used) equally in a uniform manner, OLED aging also causes non-uniformity in the OLED display.